T cell growth factor (interleukin II) is a 15,000 dalton polypeptide which is responsible for the clonal proliferation of normal T lymphocytes during the immune response. This small polypeptide is inducible by mitogen or antigen in normal human lymphocytes and recent evidence indicates that it may control the replication of certain human malignant T cells. Several years ago we found that production of T cell growth factor (TCGF) in normal cells could be completely inhibited by glucocorticoid, suggesting that glucocorticoid may be effective in treating certain human leukemias because of their effects on TCGF production.